1. Field of the Invention
The present invention generally relates to a surface acoustic wave device and a method of producing the surface acoustic wave device, and more particularly, to a surface acoustic wave device having a surface acoustic wave element sealed therein and a method of producing the surface acoustic wave device.
2. Description of the Related Art
As electronic apparatuses with higher performances have become smaller in size, electronic devices to be mounted to such apparatuses are also expected to be smaller and have higher performances. Especially, surface acoustic wave (SAW) devices to be used as electronic parts such as filters, delay lines, and oscillators for electronic apparatuses that transmit or receive electric waves, are employed in the radio frequency (RF) units of cellular phones and communication devices, so as to restrict undesired signal transmission and reception. As cellular phones and communication devices with ever higher performances are rapidly becoming smaller, those SAW devices are expected to be smaller in package size and have higher performances. Furthermore, as there is a rapidly increasing demand for SAW devices that can be used in more various fields, the production costs are expected to be lower.
FIGS. 1A and 1B illustrate a SAW device 100 that employs a conventional SAW element. Such a SAW device is disclosed in Japanese Unexamined Patent Publication No. 8-18390, particularly, in FIG. 4 of the publication. FIG. 1A is a perspective view of the SAW device 100. FIG. 1B is a section view of the SAW device 100, taken along the line F—F, of FIG. 1A.
As shown in FIG. 1A, the SAW device 100 includes a package 102 made of ceramics, a metal cap 103 that seals the opening of a cavity 109 formed in the package 102, and a SAW element 110 that is mounted in the cavity 109. As shown in FIG. 1B, the package 102 has a three-layer structure in which three substrates 102a, 102b, and 102c are laminated. Electrode pads 105, wire patterns 106, and foot patterns 107 are formed on the three substrates 102a, 102b, and 102c, respectively. The SAW element 110 has comb-like electrode or interdigital transducers (IDTs) on a first principal surface (the upper surface) of a piezoelectric substrate 111. The piezoelectric substrate 111 has a second principal surface fixed onto the bottom surface of the cavity 109. The second principal surface is the opposite surface of the piezoelectric substrate 111 from the first principal surface. In short, the SAW element 110 is face-up mounted in the cavity 109. Electrode pads 114 formed on the SAW element 110 are electrically connected to the electrode pads 105, which are exposed to the inside of the cavity 109, through metal wires 108. In other words, the SAW element 110 is connected to the package 102 by wire-bonding. The metal cap 103 is fixed onto the upper surface of the package 102 with a joining material such as solder or resin (a washer 104), so that the cavity 109 can be hermetically sealed.
Also, a small-sized SAW device can be realized by flip-chip mounting a SAW element in a face-down state on a die-attach surface. Japanese Unexamined Patent Publication No. 2001-110946 discloses such a technique. FIGS. 2A and 2B illustrate a SAW device 200. FIG. 2A is a perspective view of a SAW element 210 to be mounted on the die-attach surface. FIG. 2B is a section view of the SAW device 200, taken along the line corresponding to the line F—F of FIG. 1A.
As shown in FIG. 2A, the SAW element 210 has a piezoelectric substrate 211 as a base substrate. IDTs 213 and electrode pads 214 are formed on a first principal surface (the upper surface) of the piezoelectric substrate 211, and the IDTs 213 and the electrode pads 214 are electrically connected with wire patterns. As shown in FIG. 2B, electrode pads 205 are formed on the bottom surface (the die-attach surface) of a cavity 209 formed in a package 202. The electrode pads 205 are positioned with respect to the electrode pads 214 of the SAW element 210. The SAW element 210 is flip-chip mounted onto the die-attach surface, with the IDTs 213 and the electrode pads 214 facing the die-attach surface (this is referred to as a face-down state). Here, the electrode pads 214 are bonded to the electrode pads 205 with metal bumps 208, so that the electrode pads 214 and 205 are electrically and mechanically connected. The electrode pads 205 are electrically connected to foot patterns 207 through via wires 206 penetrating the bottom substrate of the package 202. The foot patterns 207 are formed on the bottom surface of the package 202. Signals are inputted and outputted through the foot patterns 207, and predetermined electrode pads are also grounded through the foot patterns 207. A metal cap 203 is bonded to the opening of the package 202 with a washer 204, so that the cavity 209 is hermetically sealed.
FIGS. 3A and 3B illustrate a duplexer that includes a SAW device having the same structure as one of the above SAW devices 100 and 200. In the example structure shown in FIGS. 3A and 3B, a SAW device having the same structure as the SAW device 100 shown in FIGS. 1A and 1B is employed. FIG. 3A is a section view of a duplexer 300, taken along the line corresponding to the line F—F of FIG. 1A. FIG. 3B is a top view of a SAW element 310.
As shown in FIG. 3A, the duplexer 300 has a SAW element 310 mounted on a package 302. Further, the duplexer 300 includes a substrate that has a matching circuit mounted thereon (the substrate will be hereinafter referred to as the matching circuit substrate 321), and a main substrate 322 that sandwiches the matching circuit substrate 321 with the package 302. The matching circuit substrate 321 is provided on the bottom surface of the package 302, and includes phase line paths. As shown in FIG. 3B, the SAW filter 310 includes a transmission filter 310a and a reception filter 310b. The transmission filter 310a and the reception filter 310b each includes IDTs 313 that are connected in a ladder-like fashion. The IDTs 313 are connected to electrode pads 314 through wire patterns 315.
When the SAW filter exhibits a rapid change in temperature in the above described structure, the size of the spontaneous polarization in the crystalline structure changes, and electric charges are generated on the surface of the piezoelectric substrate. In short, a pyroelectric effect occurs on the surface of the piezoelectric substrate. The electric charges are accumulated in the metal patterns (the IDTs, the electrode pads, the wires, and the like) formed on the surface of the piezoelectric substrate. As a result, sparks are caused between the metal patterns, more particularly, between the IDTs, and such sparks might damage the SAW element.
So as to solve this problem, a piezoelectric substrate having a crystalline power (or a discharging power) to reduce the amount of electric charges accumulated on the surface may be employed. Such a technique is disclosed in Japanese Unexamined Patent Publication No. 11-92147.
In the metal patterns formed on a piezoelectric substrate, however, the difference in the amount of accumulated electric charges is very large between grounded electrodes and ungrounded electrodes (or floating patterns). The discharging power of the piezoelectric substrate may be increased to eliminate such a large difference. However, a high discharging power leads to a great input signal loss and poorer filter characteristics.